Understanding the langmuir and Langmuir-Schaefer film conformation of low-bandgap polymers and their bulk heterojunctions with PCBM

Abstract

Low-bandgap polymers are widely used as p-type components in photoactive layers of organic solar cells, due to their ability to capture a large portion of the solar spectrum. The comprehension of their supramolecular assembly is crucial in achieving high-performance organic electronic devices. Here we synthezed two exemplar low-bandgap cyclopentadithiophene (CPDT):diketopyrrolopyrrole (DPP)-based polymers, with either a twelve carbon (C12) or a tri etyleneglycol (TEG) side chains on the DPP units (respectively denoted PCPDTDPP_C12 and PCPDTDPP_TEG). We deposited Langmuir-Schaefer films of these polymers blended with the widely used electron donor material [6,6]-phenyl-C61-butyric-acid methyl ester (PCBM). We then characterized the conformational, optical and morphological properties of these films. From the monolayers to the solid films, we observed distinct self-organization and surface properties for each polymer due to the distinct nature of their side chains. Emphasizing their attraction interactions with PCBM and the phase transitions according to the surface pressure. The elements amount on the surface, calculated through the XPS, gave us a good insight on the polymers' conformations. Through UV-visible absorption spectroscopy, the improvement in the PCPDTDPP film ordering upon PCBM addition is evident and we saw the contribution of the polymer units on the optical response. Chemical attributions of the polymers were assigned using FTIR Spectroscopy and Raman Scattering, revealing the physical interaction after mixing the materials. We showed that it is possible to build nanostructured PCPDTDPPs films with a high control of their molecular properties through an understanding of their self-assembly and interactions with an n-type material.